1. Field of the Invention
This invention relates to methods of fabricating etch resistant masks and more particularly relates to a method of directly using a first exposed resist layer as a photoexposure mask for a second resist layer. Such etch resistant layer are useful in the manufacture of integrated circuits and other microminiature electronic components.
2. Description of the Prior Art
Etch resistant masks are commonly fabricated in the manufacture of integrated circuits and other microminiature electronic components. In this fabrication process a radiation sensitive layer of resist material is coated on a substrate and patternwise exposed to actinic radiation such as visible or ultraviolet light, x-rays, nuclear radiation or electrons. The irradiated regions of the resist layer suffer a chemical change which makes them either more soluble (positive resist) or less soluble (negative resist) than the non-irradiated regions. A developer is then used to preferentially remove the more soluble regions, which are the irradiated regions in a positive resist and the non-irradiated regions in a negative resist. The substrate may then be subjected to a selective processing step through the openings or windows in the resulting mask, for example by etching or deposition.
Because the size of semiconductor devices is a factor in the ultimate speed of integrated circuits as well as in the initial and operational cost thereof, intensive efforts are being made to reduce the size of individual components and to increase the packing density of integrated components. Size reduction is limited, however, by the accuracy with which etch resistant masks can be fabricated and positioned. Because certain processing steps such as electroplating, reactive ion etching and lift-off require a relatively thick mask, the limiting factors in a multilevel fabrication process often are the resolution and aspect ratio (thickness of mask divided by the minimum practical linewidth at that thickness) which can be achieved during fabrication of thick etch resistant masks. Resolution and aspect ratio are limited in part by the choices of resist material and actinic radiation and in part by the type and resolution of the exposure system.
O-quinone diazide sensitized phenol-formaldehyde resist is a postive resist in common use today and is composed of a base soluble polymer such as phenol-formaldehyde novalak resin and a photoactive compound such as naphthoquinone-(1,2)-diazide sulfonic acid ester sensitizer. Such resists and sensitizers are described, for example, in U.S. Pat. Nos. 3,046,118; 3,046,121; 3,106,465; 3,201,239; 3,666,473 and 4,007,047 which are hereby incorporated by reference. O-quinone diazide sensitized phenol-formaldehyde resists have high sensitivity and submicron resolution when the resist layer thickness is sufficiently small that diffraction and absorption effects do not limit resolution. Thick resist layers of this type (greater than one micron) have a low aspect ratio and a much reduced resolution due to optical diffraction and absorption effects.
When pattern exposure of a photoresist, such as a O-quinone diazide sensitized phenol-formaldehyde resist, is done by optical projection, depth of field will also limit resolution and aspect ratio unless the photoresist layer is thin. While it is possible to avoid the depth of field problem by forming a narrow photon beam and computer controlling it to directly write a pattern onto a resist layer, this is generally not practical becuase it involves precision manipulation of an optical lens system, which is very slow and impractical. It is practical to use an electron beam in place of the photon beam but resolution is then limited instead by scattering effects. Exposure by contact printing also avoids the depth of field problem but it involves other disadvantages. Contact printing tends to scratch masks, so that contact masks have a short life and should be more durable than masks fabricated for projection printing. Proximity printing rather than contact printing extends the life of the mask but diffraction effects are worse. Diffraction effects may be reduced somewhat by reducing the wavelength of the exposure light but this type of improvement is ultimately limited by the light sensitivity and absorption characteristics of the resist. Because of these problems it has not been practical generally to fabricate high resolution thick masks from diazo sensitized phenol-formaldehyde.
Another type of positive resist in common use today is composed of certain radiation degradable alkyl methacrylate polymers. Such resists and their use are described for example in U.S. Pat. Nos. 3,538,137; 3,934,057 and 3,984,582 which are hereby incorporated by reference. Alkyl methacrylate polymers such as polymethyl methacrylate and copolymers of methyl methacrylate and polymethyl methacrylate are typically patternwise exposed by forming a narrow beam of electrons and computer controlling the beam to directly write a desired pattern onto the layer. The resolution of such a resist system is not limited by diffraction effects and direct writing speed capability is sufficiently high to be practical. In thin layers a resolution of a fraction of a micron is feasible with present technology. However, an electron beam inherently scatters or spreads in a thick layer of such radiation degradable polymers so that a low aspect ratio results. While 0.5 micron resolution is possible with a 0.25 micron thick layer of such material, resolution already drops to about one micron when the layer is 0.4 micron thick.
Alkyl methacrylate polymers are degradable with other high energy radiation as well. X-ray radiation in the 5A to 50A range produces a particularly sharp edge and very high aspect ratio is possible with this type of radiation. However, computer controlled direct writing with x-ray radiation is not practical with current technology and projection printing with x-ray radiation is not technically feasible. Contact printing and proximity printing can be done with x-ray radiation but good materials for fabricating suitable masks are not available. A gold pattern on mylar has been used for x-ray contact printing but it is difficult to form a thin layer of mylar which is uniform and defect free. Thin mylar sheets are also stretchable which makes dimensional control a further problem.
Alkyl methacrylate polymer resist layers may also be selectively degraded by patterned exposure to deep ultraviolet light of wavelength less than 300A. Direct writing is again not practical while projection exposure limits resolution in thick layers due to depth of field limitations. Contact exposure masks may be formed of chromium but thin chromium layers are difficult to form uniformly without creating pinholes. In addition, chromium layers cannot be stripped from a substrate without damaging the substrate. While contact masks of chromium are reasonably durable (for a contact mask that is) fabrication thereof requires several steps, including the fabrication of an etch resistant mask on top of the chromium layer. Such complexity is justified only where the mask will be used a large number of times. Chromium masks also produce a further difficulty due to the optical opacity of such masks, namely that such masks are inherently more difficult to align properly than an optically transparent mask such as iron oxide. Semitransparent masks employing a silver halide emulsion cannot be used because of low resolution due to the large grain of the image. Semitransparent diazo dye masks are not practical because they are very difficult to form properly and are not easily stripped. Furthermore the optical performance of such masks is unknown in the deep ultraviolet range.
It is an object of the present invention to provide a practical method for fabricating a thick etch resistant mask with high aspect ratio and resolution.
A further object is to avoid the use in such a method of an opaque contact mask which is difficult to align.
Another object is to reduce the number and complexity of the steps in such a fabrication process.
A still further object is to provide a practical method for fabricating a thick etch resistant mask having high aspect ratio and resolution when very few or only one such mask is to be made.
It is also an object of this invention to fabricate a thick etch resistant mask having high aspect ratio and resolution without forming an in contact mask of chromium.
Another object is to employ projection printing for resist exposure without having the ultimate aspect ratio and resolution limited by the depth of field of the projection printing system.
A still further object of the present invention is to use an optical resist exposure system while reducing diffraction effect upon ultimate aspect ratio and resolution.